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Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/5.9781600865206.0195.0199 | Book DOI: 10.2514/4.865206
......._,...._"""" Purchased fi"om American Institute of Aeronautics and Astronautics
PANEL DISCUSSION: AIRCRAFT NOISE PREDICTION
J. P. Raney, NASA Lan~ley Research Center: The purpose of
the aircraft noise predictlon session was to focus upon the
requirements for development of a generally accepted computer
program for noise prediction. We are interested in establishing a mechanism that will tell us when we have a satisfactory
consensus for a particular technology area. There is, among
other things, a great need for advances in analytical modeling,
which at the present time is in its infancy. The greatest
overall need is to develop and to verify experimentally analytical methods of modeling noise generating mechanisms.
Flow field. The first step, then in improving the ability
to predict noise. is to build a consistent and complete aerothermodynamic model for the aircraft engine. This model requires a general thermodynamic balance of the different stages'
of the engine cycle, a description of the turbulent atmosphere
being drawn into the engine inlet, the flow and flow gradients
in the engine ducts and blade rows, and the wall boundary
layers and blade wakes. When this information is developed,
the work on the acoustics problem of the engine may begin.
Fan noise. The details of the flow field discussed above
are especially important for fan noise prediction. Large-scale
atmospheric turbulence is drawn into the engine inlet, causing
a nonuniform axial flow into the fan blades. As the blades
rotate, this nonuniform flow causes unsteady loads on the
blades due to the varying angle-of-attack. These unsteady loads
radiate dipole noise in harmonics of the blade passage frequency. They also generate broadband noise due to the random
fluctuations of the blade load amplitudes and phases. This
noise caused by inlet flow distortion has been identified as
a key technology area for which research is needed. The understanding of both the fluid mechanics and the acoustics of the
inlet flow distortion problem is necessary for advancing the
state of the art of fan noise prediction.
Combustion noise. The unsteady combustion process in the
engine generates a low-frequency noise which sometimes has been
confused with low-frequency jet noise. The available prediction theory for this noise is empirical in nature and does not
195
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Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/5.9781600865206.0195.0199 | Book DOI: 10.2514/4.865206
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Purchased from American Institute of Aeronautics and Astronautics
DISCUSSION: AIRCRAFT NOISE PREDICTION
account for the fact that this noise must be carried through
the turbine and exhaust nozzle before it is radiated to the
far field. In order to understand this phenomenon better, a
good understanding of the flow through the turbine and exhaust
nozzle and the effect of this flow on the combustion noise
transmission are required. Again, basic thermodynamics and
fluid mechanics are an inseparable part of the acoustic prediction problem.
Turbine noise. like combustion noise, turbine noise
presently is predicted by empirical formulas which account for
only the gross variables of the problem. The few analytical
models which have been attempted use concepts similar to fan
noise in which the blades are replaced by concentrated dipoles
which represent the unsteady blade loads. However, such models
may be completely inappropriate in a turbine with high solidity
stages of highly cambered airfoils. The presence of many
stages in the turbine greatly attenuates the sound of all but
the last stage so that the sound generation and transmission
process in the turbine is quite complicated. A fundamental
approach based on realistic models of the turbine flow is
needed for turbine noise prediction. Turbine noise radiation
is influenced also by the unsteady flow field of the jet. Tones
generated by the turbine are transformed into broadband noise
as they radiate through the unsteady turbulent jet flow. This
process has been called "haystacking" because of the characteristic shape of the broadband noise which results from this
process. In turbine noise, an understanding of this effect of
unsteady turbulent flow on sound propagation is required for
improvement of our predictive ability.
Duct acoustics. Noise from sources inside of the CTOl
engine may be attenuated by the addition of sound-absorbing
material inside the nacelle. Very precise complex analytical
models of duct transmission have been developed; however, these
analyses are for idealized duct and flow models. In an actual
engine, the duct wall boundary layer significantly affects the
attenuation of the sound, especially in the inlet. Thus, a
realistic description of the flow is necessary before a prediction can be made. Also, these precise analytical models of
duct transmission are based on a linear boundary condition, the
duct wall impedance. It is known that the acoustic materials
used in engine nacelles are nonlinear at the sound intensities
which occur in these engines and that the flow over the
materials has a major influence on this property. Therefore,
a primary area where work is needed in duct acoustics is the
modeling of duct problems with realistic nonlinear boundary
conditions.
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Purchased rrom American Institute of Aeronautics and Astronautics
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/5.9781600865206.0195.0199 | Book DOI: 10.2514/4.865206
DISCUSSION: AIRCRAFT NOISE PREDICTION
197
Much of the work in duct acoustics in the past 10 years
has been developed using the modal theory of sound transmission.
Unfortunately, researchers have carried idealized transmission
analyses to extremes, making predictions of attenuation based
on a single-mode assumption. Attempts also have been made to
generate pure modes in the laboratory in order to verify their
properties. Real engine noise sources, however, are always
represented by a large number of modes interacting in a complex
manner, and this must be accounted for in any realistic prediction attempt. The modeling of real sources as well as real
boundary conditions is necessary for improving the state of
the art in duct acoustics. Also, modal theory is not essential
to the duct propagation phenomenon; it is only a tool. Other
tools are now being considered. One which shows promise is
the finite element method which has reached a high level of
development in the field of structural analysis. Just as duct
acoustics modal theory evolved from electrical transmission
line theory, the finite element techniques of structural analysis may be developed into an acoustic transmission line theory
which will be competitive with modal theory in the prediction
of duct acoustic effects. The development and comparison of
both of these methods is a fertile area for further research.
Jet noise. Jet noise is one of the oldest subject areas
of concern in the overall CTOl noise problem. In spite of
this, our predictive capability for real-world jet noise problems is not well developed. Presently, an empirical formula
is being used by NASA for jet noise predictions. The difficulty with empirical formulas is that each is derived to
represent only a certain set of data. The SAE A-2l committee,
for example. has an empirical jet noise prediction formula
which no doubt represents their data, but NASA and SAE predictions are different. They are different because they are
based on different data. To eliminate these differences, it is
necessary to develop a unified data base for jet noise. The
data which are entered into this base should be required to
meet certain standards e ~~~li~herl hy thp peer group of experimentalists in this field. These experimental standards will
rule out certain carelessly conducted experiments and define
the subset of jet noise data which will be included in the jet
noise data base. The gathering of this information also will
define additional experiments to be carried out. Then, if an
empirical correlation is made. only one formula may be considered "best." This is the formula with the least variance
of the estimate.
A unified data base also will serve to define the direction that analytical work in jet noise should take. If
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Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/5.9781600865206.0195.0199 | Book DOI: 10.2514/4.865206
198
Purchased rrom American Institute of Aeronautics and Astronautics
DISCUSSION: AIRCRAFT NOISE PREDICTION
empirical formulas are inadequate, analytic models based on
Lighthi11 's, Phillip's, or Lilley's equation may be used. In
these partial differential equations, the source terms must be
modeled by some assumed turbulent flow. Here again the basic
fluid mechanics of turbulent flow enter the picture. It is
necessary to compare a sequence of models for the source terms
in both Lighthi11 's, Lilley's, and other jet noise formulations
to see which provides the least variance of the estimate
against a unified data base. When comparing the solutions to
partial differential equations, however, the accuracy of the
prediction is not the only criterion which may be cited to
determine which of several methods may be best. The cost of
prediction, as judged by computation time, for example, is
another factor which must be considered. Perhaps the most
important consideration of all is, does the predictive equation
provide a realistic method for achieving noise reduction? All
of these factors must be considered in arriving at a "best" jet
noise prediction method.
Airframe noise. Besides the engine, the various components
of the airframe may radiate significant amounts of noise during
the landing approach of a CTOL aircraft. Here again we are at
the empirical formula level in our state-of-the-art prediction
capability. Presently, we use a formula developed for aircraft
in the "clean" configuration from a limited but well-defined
data base. It is recognized that the extension of flaps and
landing gear will increase the airframe noise by 10 dB or even
more, so that the present prediction method is an interim
device used for order-of-magnitude estimations. A promising
empirical approach which accounts for the effects of flap extension is the drag element noise theory. In this theory, each
airfoil is assumed to produce a noise in proportion to the cube
of its drag coefficient. This theory is related to the analytic
theory of edge noise which is probably the dominant component
of airframe noise. Edge noise theory, however, depends on the
turbulent flow conditions at the trailing edge of an airfoil,
so we see a fundamental dependence of the acoustics of airframe
noise on the fluid dynamics of the airfoil. Research in this
field must proceed along a consistent path using valid models
of the turbulent boundary layers in comparably valid acoustic
theories. Experiments in edge noise must simultaneously study
the fluid dynamics of the turbulent flow and the noise radiation. Precise flight tests also are required to validate
empirical theories such as the drag noise theory.
Noise propagation. CTOL noise must propagate over large
distances before it reaches the community. The character of
the noise is modified during this propagation process due to
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Purchased rrom American Institute of Aeronautics and Astronautics
Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/5.9781600865206.0195.0199 | Book DOI: 10.2514/4.865206
DISCUSSION: AIRCRAFT NOISE PREDICTION
199
the dependence of attenuation on such factors as frequency,
temperature, and humidity. Fortunately, available prediction
methods account for the more important absorption processes,
classical absorption and molecular absorption, if the ambient
atmospheric conditions are known along the ray from the source
to the observer. There remains some controversy about the
effects of atmospheric turbulence on propagation which must be
resolved by careful experimental work. A more important research area relates to the effects of ground absorption on the
propagation of sound. There is a strong theoretical base for
prediction of ground absorption, but these prediction methods
depend on the impedance of the earth surface which is seldom,
if ever, known. Thus, careful studies are required to develop
a data base of ground impedance data for the various types of
terrain which are involved in the aircraft noise propagation
problem. The development of these data by careful experiments
will greatly improve the accuracy of our noise prediction
methods.